Seems like you want to go NA. If thats the case, on a GSR, a mild bottom end rebuild would include JDM Type R pistons, or USDM PR3 pistons, along with all the new bearings, rings, etc. With a GSR head and block, that would give you an 11.3:1 CR.

Up top, ITR/CTR cams would compliment that perfectly, with the valvetrain of your choice. This would give you potential for 180-190 to the wheels with a good tune.

This is what I'm going to do.